I want to migrate me and my friend group from FB chat to a more secure/privacy-minded chat group.

Almost everyone uses Linux altough there are some Windows users. Being able to chat from a PC and a smartphone is also pretty important.

Any ideas what to use?

There are two threads already dedicated to this subject:
Tox: https://nixers.net/showthread.php?tid=2110
Messaging: https://nixers.net/showthread.php?tid=2067